Ok 7 hours you say. Lets check some numbers:
7 hours are 420 minutes. Any by assuming you did the standard way of getting a larvling (2 mins for 1 larvling to spawn), that would be 210 larvlings. There are currently 19 Kuva weapons, and the chance to drop a specific weapon is 1/19, and not getting the desired weapon is 18/19.
So by combining the possibility of not getting your desired kuva weapon (18/19) to the power by the amount you have tried to spawn a kuva weapon (210 larvlings).
So in total (18/19)^210 = 0.0000117 = 0.00117 %, which is the total % of you not getting the kuva weapon of your choice after all the runs.
Usually, after 14 tries you should have a 50 % chance of your weapon beeing in the drop pool, and normally 90 tires has a 99 % chance of your weapon beeing in the drop pool.
I dunno man a 5 % chance of getting a weapon you want is not that bad in a looter game, especially when you compare it to other drop stuff.